Biacore

Surface plasmon resonance (SPR)

The technique

Surface plasmons are surface electromagnetic waves that propagate in parallel along a metal/dielectric (or metal/vacuum) interface. Since the wave is on the boundary of the metal and the external medium (air or water, for example), these oscillations are very sensitive to any change of this boundary, such as the adsorption of molecules to the metal surface.

To describe the existence and properties of surface plasmons, one can choose from various models. The simplest way to approach the problem, is to treat each material as a homogeneous continuum, described by a dielectric constant. With the terms of this description for electronic surface plasmons to exist, the real part of the dielectric constant of the metal must be negative and its magnitude must be greater than that of the dielectric. This condition is met in the IR-visible wavelength region for air/metal and water/metal interfaces (where the real dielectric constant of a metal is negative and that of air or water is positive).

Applications

The primary use is in the quantitative measurement of intermolecular interactions, especially between proteins and other molecules.

In their simplest form, SPR reflectivity measurements can be used to detect DNA or proteins binding to a target by the changes in the adsorption of the target molecule to a metal surface.

The molecules are bound to a chemically modified chip. The chip may be coated with caboxymethyl groups (which will bind positively charged protein), streptavidin (binds biotinylated molecules), or NTA (for His-tagged proteins). Proteins, nucleic acids, carbohydrates and low molecular weight molecules may be attched to the surface.

Questions that can be addressed wtih teh Biacore 3000 include:

How specific is the binding between tow molecules?

How much of a given molecule is present and capable of binding to another?